C-13 uptake experiments were carried out at a station off the pleat coast of Hokkaido Island, Japan, during the period between late winter and spring bloom in 1997. The composition of newly biosynthesized particulate fatty acids was determined using a C-13 tracer and gas chromatography/mass spectrometry (C-13 GC/MS), and compared with that of particulate matter (seston). Two fatty acid compositional ratios revealed that diatoms biomass (16:1(n-7)/16:0, Sigma C-16/Sigma C-18) were strikingly high in new cells compared with seston on 17 April, coinciding with the dominance of fucoxanthin. In this study, the Polyunsaturation Index (a measure of the percentage of C-16 fatty acids that are polyunsaturated) in new cells is generally close to that in seston, while there was a notable discrepancy between new cells and seston in surface (10 m) waters on 17 April. This large difference between new cells and seston is ascribed to: (1) an increase of newly synthesized storage lipids (mainly 16:0 and 16:1) induced by nitrogen limitation in the top 10 m of the water column; (2) an addition of earlier synthesized phytoplankton cells (high Polyunsaturation Index at logarithmic growth stage) and non-phytoplankton components (detritus and bacteria and zooplankton) into seston. The highest Polyunsaturation Index of C-16 fatty acids in seston (33%) occurred at 100 m water depth. This may be caused by "settling planktonic aggregates" or "diatom resting spores". In the present study, it is reasonable to suggest that the Polyunsaturation Index of C-16 fatty acids can be a useful indicator for the ecophysiological state of marine diatom populations. (C) 2000 Elsevier Science B.V. All rights reserved.